Fiber chromatic dispersion has played an important role in the design of optical fiber systems for more than a decade. Until the advent of the erbium-doped fiber amplifier, the systems were more or less linear. Hence, it was only the integrated dispersion over a fiber span that influenced system performance. As the need to satisfy the demand for transmission capacity over greater distances has led to more sophisticated processing of optical signals, dispersion management--in which a dispersion "map" is chosen to minimize and/or harness the effects of fiber nonlinearities, has becoming an increasingly important tool.
In dispersion-shifted fiber (DSF), dispersion is known to vary as a function of location in the fiber. In a paper by K. Inoue entitled "Four-Wave Mixing in an Optical Fiber in the Zero-Dispersion Wavelength Region", J. Lightwave Technol., Vol. 10, pp. 1553-1561 (1992), for example, it was reported that when a 10-km section of DSF was cut into four 2.5-km segments, the average dispersion zero wavelength for the segments for the segments varied by at least 1 nm--a significant deviation for some applications. Accordingly, a reliable map of chromatic dispersion can not be obtained merely by measuring the average dispersion in the fiber span.
It has therefore been proposed to map the distribution of chromatic dispersion along a fiber span using a Rayleigh backscattering technique. This destructive technique, which relies on the dependence of the dispersion zero on the fiber core size, is described in an article by M. Ohashi and M. Tateda entitled "Novel Technique for Measuring the Distributed Zero-Dispersion Wavelength of Optical Fibers", Electron Letters., 29, 426-428 (1993). If the doping of the fiber preform does not change over its length, then changes in the dispersion zero can be inferred from changes in the core size. Changes in core size are estimated using optical time domain reflectometry (OTDR) to determine the capture ratio for Rayleigh backscattered light. By summing OTDR measurements taken in opposite directions, the effects of fiber loss are removed and changes in the capture ratio are observed and used to determine variations in the fiber dispersion zero.
Recently, a non-destructive dispersion measurement method was described that determined the local dispersion zero from modulation-instability-induced gain at wavelengths longer than the dispersion zero. A strong pump pulse of wavelength .lambda..sub.p and a weak signal pulse of wavelength .lambda..sub.s are injected simultaneously into a test fiber with the difference between .lambda..sub.p and .lambda..sub.s being about 5 to 10 nm. The backscattered signal light is observed through OTDR. When the pump wavelength is near the dispersion zero, but in the anomalous dispersion region, the modulation instability will provide gain for the probe pulse--gain that can be observed in the OTDR trace. Thus, reduction in the slope of the OTDR at a particular distance into the fiber indicates that the pump is experiencing anomalous dispersion at that point in the fiber. To map fiber dispersion, the pump and probe wavelength are swept, maintaining a constant separation, .lambda..sub.p -.lambda..sub.s, and the resulting OTDR traces are recorded. The dispersion zero of a particular point in the fiber is at the short-wavelength side of those pump wavelengths for which modulation-instability gain is observed. This technique has demonstrated a wavelength resolution of 0.2 nm and a spatial resolution of about a kilometer.
Yet another technique that has been proposed uses partially-degenerate four-photon mixing to determine the dispersion zero. Essentially, the mixing generates an idler wave from pump and signal waves of angular frequencies .omega..sub.p and .omega..sub.s propagating in the fiber. The power of the idler wave with frequency .omega..sub.i =2.omega..sub.p -.omega..sub.s, will be maximized when the process is phase matched, that is, when .DELTA..beta.=2.beta.(.omega..sub.p)-.beta.(.omega..sub.s)-.beta.(.omega.. sub.i)=0, where .beta.(.omega.) is the propagation constant. To first approximation, phase matching occurs when .omega..sub.p is set to the dispersion zero of the fiber. Thus, by tuning .omega..sub.p, and looking for a maximum in idler power, it is possible to measure the zero-dispersion wavelength. Distance resolution is obtained by using signal and pump pulses with widely-separated wavelengths. Specifically, the differing group velocities of the pump and signal pulses cause the pump to overtake the signal pulse (assuming the pump wavelength is near the dispersion zero of the fiber). With sufficient group-velocity dispersion and short enough pulses, the region of overlap of the pulses occurs over some useful distance. The timing of the pulses at .omega..sub.p and .omega..sub.s can then be adjusted so that this overlap occurs at some desired point within the fiber.
Each of the above-described techniques permits measurement of the wavelength of zero dispersion. Disadvantageously, such techniques require extensive data-gathering over a considerable wavelength range, so that the measurements for just one dispersion map take a long time. Moreover, access is required to both ends of the fiber under investigation.